We have a job where we replaced an amplifier for a client and added a pair of speakers. The original system had a Niles SSVC6 speaker selector, a 100 watt Pararsound amp and 4 pairs of speakers. The new amp is a Parasound 250 watt model and we added one more pair of rock speakers (8 ohm load with new speakers). When we fired everything up, we started getting a knocking speaker selector and all of the speakers sounded distorted. The speaker selector is on its max setting (5/6 pairs).

Niles thinks that the speaker selector is seeing impedance above 8 ohms. We'll be testing that today as all of this happened late at night and we had to beat it but any ideas? The speaker selector is rated at 100 RMS so I think we may be overpowering it. Aside from a bad speaker selector (it worked fine with 4 pairs and the 100 watt amp for 15 years and started having the issue once we put in the new speakers and amp), any ideas?

Thanks in advance

I'm reluctant to use amplifiers rated for more than 100W in this application. Likely, the amplifier has a small DC offset, perhaps only during startup, but this DC offset causes the volume control autotransformers to saturate. Once saturated, the autotransformers present a very low impedance to the amplifier and it will misbehave. That "knocking" sound that you hear from the speaker selector is generated by the controls as they saturate.

There are some not so good options: Start by measuring the amplifier output with a DC voltmeter while no speakers are connected. If you measure more than about 0.1V, be very annoyed. You may be able to adjust this -- at the risk of voiding the warranty. Another approach is to add a small series resistor between the amplifier and the Niles box. Probably, something on the order of 0.5 Ohms (plus or minus) will do the job. Pay attention to the resistor's power rating and allow some air circulation.

Thx for the input!

Also, if you do that, put some wire between the resistor and the SSVC-6. We took one out of someone else's install where they had put a four ohm resistor in series with each channel, connecting the resistor directly to the SSVC-6. The resistor had gotten so hot during usage that it had mostly melted the plastic parts of the connector on the switcher.

Let's say there's no DC on the amp output: remember that a 250 watt amplifier that's not turned all the way up puts out much less than 250 watts. It might not be the amplifier power at all!

Download the instruction manual and follow the setup instructions. A particularly important step in setup is how you set the power amp volume. You will have problems if the power amp outputs more than it needs to output to satisfy the volume requirements of the system. In brief,
set the impedance control properly, per the manual;
turn the SSVC-6 controls up ALL THE WAY;
turn the power amp up enough to hear sound in all locations;
walk from room to room and find the one that is the lowest in volume;
and set the volume of the power amp for the loudest volume that you're going to want to have in that room (all other rooms will play louder so you'll have to lower their controls more than the control for this room).
If possible, at this point remove the client's ability to adjust the power amp's volume. If not, someone some day will decide that more volume is needed, and instead of turning up a room control, they'll turn up the power amp.

This guarantees that all rooms will play loud enough and the amp won't play too loud. If the thing knocks then, you simply can't use this approach.

punter16,

If you can find one, a Russound LFP-1 or LFP-1.2 will be helpful. This is a high pass filter inserted between the amplifier and the control. If you think that this looks like a pair of capacitors, you are correct.

You're right- it's rated for 100W, 200W peak power.

This is a good reason for using multi-channel amplifiers and controlling the level ahead of each channel. Regardless of the stereo amp's power, when a speaker selector/level control is used, the power is shared by all of the speakers that are in use and some of it is wasted.

I agree. This was a job that someone wired 15 years ago and there are no VCs anywhere...all speaker wires are run directly to the cabinet.

We ended up going with a less powerful amplifier and everything worked well.

Thanks all for the input!

BTW: For most practical purposes, the controls stuffed inside the box are the same controls that one would use in-wall. The saturation issues will be the same in the wall or in the box.

Saturation can also occur during strong Bass passages. This is an area where a high pass filter is helpful.

I managed Niles tech support during the 90's when speaker selectors were more widely used than they are today with the advent of wireless music system. Thumping sounds from a selector with volume controls or thumping from a volume control installed in the wall was one of the more frequent inquires by dealers and end users.

The proper remedy is the same in both instances, you need to turn the connected amplifiers volume down. The autoformers in the individual volume controls are what are make the thumping sound and it is as Buzz states, you are hearing the louder bass frequencies overload the laminated coils in the autoformer.

Also, in most cases, if a volume controls autoformer is thumping, the speakers will play too loudly when that individual volume control is turned all the way up. So lowering the amplifier volume not only solves the thumping problem, it also provides more resolution in the volume settings (e.g. the lowest setting can now play at a lower level and the highest setting plays no louder than ever needed).

This is exactly why I wrote the procedure in my previous post. It gets the quietest room as loud as you'll ever want it with the POWER AMP volume only as far up as is needed to do that.

This is especially prevalent in systems where it's easy to change the power amp's volume.

The first time I ran across this, the client's Marantz remote (it was the late 90s) had Zone 2 volume. One day we heard a thumping and went to investigate. Someone had turned the wall volume control down to about 1, then turned up the power amp output to get sound of of that speaker. The cure was to take power amp volume control off the programmable remote, then turn up the wall control and adjust the power amp only as high as was needed.

I've overcome people's tendency to try to increase power amp volume by pointing out that speaker systems like this bear a resemblance to watering systems. The power amp is the city's water supply and each volume control is a faucet. If your faucet is turned on to 1 on a scale of 10, you don't call the city and tell them to increase the system pressure. You open the faucet more. ONLY when the volume controls/faucets are all the way up do you even consider turning up the power amp/asking the city for more water pressure. This seems to get it across.


This gets a loud thumping AMEN.

The issue of resolution is a great one but it takes understanding the rest of this to even think of it! Furthermore, individual room volume controls can do better than a switch like this Niles switch, because you can set an outdoor speaker volume control to be, say, X4, while the powder room volume control is set to X16. That way, more power goes to the outdoor speaker, which needs it, than to the powder room, which is a small enclosed space. Without being able to make this adjustment in the setup, the powder room volume might be overbearing at just 3 or 4 out of 10.